Since the invention of Xerography (which means "dry writing" in Greek) by C. Carlson in 1938, new facilities utilizing this technique such as Xerox copier, laser printer and optical printer have provided inexpensive, convenient and fast services of copying documents and played important roles in office automation.
The focus of the Xerography technique resides in the photoreceptor which is an optical element electrically insulative before exposure under light and becomes electrically conductive after exposure. The Xerographic process comprises mainly five steps, namely, (1) charging, (2) photodischarging, (3) image transfer, (4) development and (5) cleaning. In order to obtain printed images of high quality, the photoreceptor must have high charge acceptance, low dark conductivity and fast photoconductivity (i.e., high sensitivity).
Photoreceptors can be classified as inorganic or organic. Due to the advantages of low production cost, non-toxicity and high flexibility, organic photoreceptors (OPC) have replaced inorganic photoreceptors and come into prominence in commercialized photoreceptors.
The structure of photoreceptors may be classified as (1) mono layer type, (2) functionally separated laminated type, and (3) microcrystalline distribution type. The functionally separated laminated layer type is the most preferred because it contains separated charge generation layer (CGL) and charge transport layer (CTL) and thus is highly flexible in the selection of materials for each layer. The characteristics and requirements may be adjusted as desired independently in CGL or CTL. This type of photoreceptors are predominant among the present photoreceptors.
The functionally separated laminated type photoreceptors are generally composed of a conductive support, a charge generation layer and a charge transport layer. An optional barrier layer may be inserted between the conductive support and the charge generation layer. In the production of photoreceptors of this type, a charge generation layer composed of a charge generation material and a polymer binder is coated on a conductive support and then a charge transport layer composed of a charge transport material and another polymer binder is coated.
Among the light sources for laser printers, the helium or neon laser has the wavelength of 633 nm, and the wavelengths of semiconductor lasers (such as arsenic aluminium gallium laser) is 780 nm or longer. Light sources having such wavelength are generally classified as "near infrared" light. Because semiconductor lasers can be installed in a minimum construction, are highly reliable, and can operate at high speed, they are most commonly used. In conformity with the semiconductor lasers, the charge generation material used in the OPC for semiconductor laser printers must possess high sensitivity to lights of 780 nm or higher wavelength.
U.S. Pat. No. 4,426,434 discloses a process for producing OPC in which a conductive support is vacuum deposited by chloroaluminium phthalocyanine or chloroaluminium monochlorophthalocyanine and treated with solvent vapor to produce an OPC having improved sensitivity to light within the range of near infrared wavelengths. However, the process involves a step of vacuum-deposition which requires expensive apparatus and needs very a long processing time. The cost for the process is therefore very high, rendering the implementation of the process nearly impractical.
U.S. Pat. No. 3,824,099 discloses that squarylium pigment is sensitive to wavelengths of near infrared range. The squarylium pigment is generally prepared by an "acid route" in which one equivalent of squaric acid and two equivalents of N,N-dialkylanilines derivatives is reacted in an azeotropic solvent. The synthesis reaction is quite simple and has high yield. However, the squarylium synthesized by this process has high dark conductivity and low charge acceptance when used as the charge generation material for photoreceptor. To minimize the influences of these two drawbacks, the thickness of the charge generation material layer must become very thin. Under such thickness, the ability of the photoreceptor to absorb incident lights will be lowered and a large amount of incident light will be reflected, resulting in severe interference and great degradation in the quality and resolution of the printed image or characters.
Copper phthalocyanines pigments have high coloration value, photo-resistance, heat resistance and chemical-resistance and are non-toxic and thus are commonly used as green-blue pigment. The pigments are known to exist in eight crystalline forms, i.e., alpha-, beta-, epsilon-, gamma-, delta-pi-, rho- and chi-types, with alpha-, beta- and epsilon- being the most prevailing. Copper phthalocyanines pigments have long been studied for use as a photosensitive material but due to their low sensitivity they have never been developed to a stage of industrial implementation.